The infrared spectrum covers a range of wavelengths longer than the visible wavelengths, but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The infrared wavelengths extend from 0.75 micrometers to 1 millimeter. The function of an infrared detector is to respond to the energy of a wavelength within some particular portion of the infrared region.
All materials generate radiant energy having characteristic wavelengths within the infrared spectrum depending on the temperature of the material. Many current infrared image detection systems incorporate arrays with large numbers of discrete, highly sensitive detector elements, the electrical outputs of which are connected to signal processing circuitry. By analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and track sources of infrared radiation.
Contemporary subarrays of detectors may be sized to include a signal array with 256 detector elements on a side, or a total of 65,536 detectors, the size of each square detector being approximately 0.009 centimeters on a side, with 0.00127 centimeters spacing between detectors. Such a subarray would therefore be 2.601 centimeters on a side. Interconnection of such a subarray to signal processing circuitry would require connecting each of the 65,636 detectors to processing circuitry within a square, a little more than one inch on a side. Each subarray may, in turn, be joined to other arrays to form an extended array that connects to 25,000,000 detectors or more. As would be expected, considerable difficulties are presented in electrically connecting the detector elements to associated circuitry.
Though the theoretical performance of such contemporary systems is satisfactory for many applications, it is difficult to construct structures that adequately interface large numbers of detector elements with associated signal processing circuitry in a practical and reliable manner. Consequently, practical applications for contemporary infrared image detector systems have necessitated further advances in the areas of miniaturization of the detector array and accompanying circuitry. Such miniaturization has necessitated improvement in the mechanical and electrical interfacing of detector element arrays to processing circuitry such as that found in Z-plane modules. Mechanical interfacing is additionally difficult when the detector material is brittle or consists of a fragile thin layer.
The Z-plane module is a layered structure containing integrated circuits which perform signal processing functions on the outputs of the detector element array. The Z-plane module extends along the Z axis, perpendicularly to the X-Y plane, i.e. the plane of the detector array. Thus, signal processing circuitry can be mounted on the focal plane, just behind the detector array, and can extend a sufficient distance in the Z direction to permit the mounting of the required components.
The outputs of detector elements typically undergo a series of processing steps in order to permit derivation of the informational content of the detector output signal. The more fundamental processing steps, such as preamplification, tuned band pass filtering, clutter and background rejection, multiplexing and noise suppression, are typically accomplished within the Z-plane module.
Less signal processing can be performed by circuitry on an x-y oriented wafer substrate than by a z-module. Such signal processing is usually limited to preamplification and multiplexing. In either case, the detector array is bump bonded to the processor. Because of the small size and the high density of these bonds, the detector array substrate must have sufficient mechanical integrity to withstand the bonding process and flexing when the detector array and the processor are operated cryogenic temperatures.
Contemporary detector interface devices, particularly for detector arrays of HgCdTe epitaxially grown on CdTe, are typically ceramic or epoxy structures which provide for the mounting of detector arrays and provide for their mechanical and electrical connection to Z-plane modules. The detector arrays are installed on the detector interface device and the detector device on the module. Two operations are needed. Both require a high degree of alignment precision. It would be more desirable to provide a detector interface device which is an integrated structure with the detector element array. Forming the detector elements directly upon a high strength, thermally stable substrate would permit the substrate to function as a detector interface device which would be suitable for maintaining the electrical interconnection of the multitude of very small, closely spaced bonds. It would also eliminate the steps of having to electrically and mechanically interface the detector elements to the detector interface device. It would therefore eliminate a bonding operation and so result in a more reliable interconnection.
As such, although the prior art has long recognized the difficulty of obtaining structural integrity and thermal stability of the electrical and mechanical interconnection between infrared detector element arrays and their associated signal processing circuitry, the proposed solutions to date have been ineffective in providing a satisfactory remedy.